Cadmium electroplating



cAnMnJM ELECTROPLATING Peppino N. Vlannes, Fort Washington Forest, Md.,and Simon W. Strauss and Benjamin F. Brown, Washington, D. C.

No Drawing. Application September 5, 1957 Serial No. sszmn 8 Claims.(Cl. 20450) (Granted under Title 35, U. S. Code (1952), sec. 266) Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates to a process for the electrodeposition of cadmiumand to new electroplating baths therefor.

Experience with cadmium plated high-strength steels has shown thatfracture may occur in service after a period of time under anessentially constant stress much lower than the yield strength. Thisphenomenon, described variously as delayed fracture or static fatigue,is known to be associated with the presence of hydrogen in the steel. Inrecent years there has been a mounting incidence of service failures inwhich cadmium electroplated high-strength steel articles fail by delayedfracture.

It has long been known that hydrogen is introduced into the steel andother metals in the electroplating with cadmium from the cyanide bath.It is a characteristic of the cyanide bath process that both hydrogenand cadmium are deposited at the surface of the cathode. Part of thedeposited hydrogen is eliminated by bubblingaway, but part of it becomesdissolved in the metal undergoing plating. The presence of thisdissolved hydrogen reduces the fatigue strength of the metal and rendersit susceptible to delayed fracture through hydrogen embrittlement.

It is an object of the present invention to provide a new process forelectroplating cadmium. It is a further object to provide a process forcadmium electroplating in which deposition of hydrogen and consequenthydrogen embrittlement of the metal is markedly reduced over thatoccurring in the cyanide bath process. It is another object to providenew electroplating baths. It is also an object to provide newelectroplating baths which are non-cyanide in composition.

We have found that the above and other objects can be accomplished inaccordance with our invention by electrodepositing cadmium from a bathcomprising an aqueous ammoniacal solution of a cadmium salt other than acyanide and an alkyl monoaminomonocarboxylic acid of 2 to carbon atomsin which the molar ratio of amino acid to cadmium is from 1:1 to 2:1 andthe pH may range up to about 11.2, preferably up to about 10, andoptimally is from about 9 to 10. In the alkaline range of pH, the aminoacid is present in the aqueous ammoniacal solution as a complex withcadmium.

It is known that the ease with which hydrogen is discharged in anelectrode process depends partly on the concentration of hydrogen in thebath, i. e., on the pH of the bath. To lessen hydrogen discharge,therefore, it is preferable to work in the alkaline range. However, overa wide range of pH, cadmium tends to precipitate fromammoniacal'solution as the hydroxide unless it is properly complexed.The presence of cadmium in the electroplating baths of our invention asa complex with the amino acid permits working in the alkaline rangeacid, etc.

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without precipitation of cadmium as the hydroxide. The presence of thecadmium-amino acid complex also contributes to obtaining a good plate.The complex formed in the ammoniacal solution between the cadmium andthe amino acid is a chelate and is thus distinguished from the complexof cadmium and ammonia which is characterized by being a salt in whichfour NH groups are attached to cadmium.

The electroplating baths of the invention can be made up in simplemanner by dissolving the cadmium salt and the amino acid in a volume ofwater or of dilute ammonium hydroxide which is slightly less than thatof the final volume of the solution and adjusting the pH and/ or volumeof the solution to the desired level by addition of concentratedammonium hydroxide and water as required. Suitable cadmium salts forpreparation of the new baths are, for example, cadmium sulfate,chloride, bromide, acetate, citrate, tartrate, etc. Suitable amino acidstherefor are, for example, glycine, /3-alanine, a amino-n-butyric acid,a-amino-isobutyric acid, [it-aminon-valeric acid, 'y-amino-n-valericacid, ,B-amino-isovaleric The concentration of c-adimum in the bath mayrange between about 0.5 and 2 molar and preferably is between about 1and 2 molar. The amino acids can be present in the bath in molarconcentrations from one to two times that of the cadimum. The molarratio of cadmium and amino acid in the bath, as stated above, may rangebetween 1:1 and 1:2, with no apparent advantage to be had by the use ofhigher ratios of amino acid to cadmium.

The composition of typical electroplating baths in accordance with theinvention are contained in the table below in which the weight of thestarting components is in grams per liter of aqueous ammoniacalsolution.

Weight Weight pH of Amino Acid Amino 30 (18 04.81350 solution AcidGlycine 305 9-10 fl-ala-nine 320 465 910 a-amino-n-butyric acid. 1509-10 a-amino-isobutyric acid. 150 190 9-10 on iron, copper andcopper-rich alloys and on other metals to be plated with cadmium. Fromthe practical standpoint, the electroplating is carried out at roomtemperature, although the process may also be conducted at temperatureswhich are considerably below or above room temperature, as low as about4 C. and as high as the boiling point of the ammoniacal solution of theamino acid-cadmium complex, with the production of a uniform, adheringdeposit of cadmium on the metal to be plated using current densities offrom .0027 to .027 amps/cm. (2.55 to 25.51 amps/ft?) An average currentdensity for operation of the electrodeposition process of the inventionis 15 amps/ft? Comparative tests involving electroplating a cylindricalnotched bar of A181 4340 steel of approximately 285,000 p. s. i.ultimate tensile strength as substrate from baths of the composition setforth in the table above and from a standard cadmium-cyanide bathdemonstrated the marked reduction in hydrogen deposition and hydrogenembrittlement which can be obtained by electroplating with the processof the invention. The baths of the .3. composition of the invention wereat near saturation with respect to the amino acid and ammoniated to theoptimum pH range. The current density therefor was 15 amps/ft. in eachcase. The current density for the standard cadmium-cyanide bath was 25amps/ft The platings were carried out at room temperature for a platingtime of 15 minutes. The plate thickness on the barrel of the specimenwas about .0007 inch.

For the delayed fracture test of the plated specimens, the load appliedwas 75% of the ultimate tensile strength of the unplated bar. Thisrepresents very severe test conditions. The steel bars were sandblastedrather than pickled before being plated, simply to eliminate theembrittlement known to be caused by pickling alone. All specimens in theseries were loaded immediately after being plated and examinedmicroscopically. Criteria for acceptable plating of the tensile barsincluded coverage by cadmium of the root of the notch as judged bymicroscopic examination of a metallographic section. The delayedfracture test used for determination of hydrogen embrittlement was thatof Raring and Rinebolt, ASTM Bulletin No. 213, pages 74-76, April 1956.From the standpoint of reduced hydrogen embrittlement, glycine anda-amino-nbutyric acid are preferred amino acids for preparingelectroplating baths of the invention.

Additives which have been heretofore employed in cadmium electroplating,such as wetting agents, e. g., iso propyl naphthalene sulfonic acid,brighteners, e. g., dextrose, polyvinyl pyrrolidine, may be employed inthe electroplating baths of the invention for particular im provementeffects in the plating.

Since the principle of the invention herein described may be variouslypracticed without departing from the spirit or scope of the invention,it is to be understood that specific embodiments appearing in the abovedescription are to be taken as illustrative and not in limitation exceptas may be required by the appended claims.

What is claimed is:

1. In the electrodeposition of cadmium from aqueous bath, theimprovement which comprises electrodepositing cadmium from acyanide-free aqueous ammoniacal bath of pH up to about 11.2 in which isdissolved an alkyl rnonoaminomonocarboxylic acid of from 2 to 5 carbonatoms and a cadmium salt in a molar ratio of from about 1:1 to 2: 1.

2. In the electrodeposition of cadmium from aqueous bath, theimprovement which comprises electrodepositing cadmium from acyanide-free aqueous ammoniacal bath of pH up to about 10 in which isdissolved an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbonatoms and a cadmium salt in a molar ratio of from about 1:1 to 2:1.

3. In the electrodeposition of cadmium from aqueous bath, theimprovement which comprises electrodepositing cadmium from acyanide-free aqueous ammoniacal bath of pH up to about 10 in which isdissolved an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbonatoms and a cadirnum salt in a molar ratio of about 2:1.

4. In the electrodeposition of cadmium from aqueous bath, theimprovement which comprises electrodepositing cadmium from acyanide-free aqueous ammoniacal bath of pH up to about 10 in which isdissolved an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbonatoms and cadmium sulfate in a molar ratio of from about 1:1 to2:1.

5. In the electrodeposition of cadmium from aqueous bath, theimprovement which comprises electrodepositing cadmium from acyanide-free aqueous ammoniacal bath of pH up to about 10 in which isdissolved an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbonatoms and cadmium sulfate in a molar ratio of about 2:1.

6. A cyanide-free aqueous electroplating bath comprising an aqueousammoniacal solution of an alkyl monoaminomonocarboxylic acid of from 2to 5 carbon atoms and a cadmium salt in the molar ratio of from about1:1 to 2:1 in which the pH is up to about 11.2.

7. A cyanide-free aqueous electroplating bath comprising an aqueousammoniacal solution of an alkyl monoaminomonocarboxylic acid of from 2to 5 carbon atoms and a cadmium salt in the molar ratio of from about1:1 to 2:1 in which the pH is up to about 10.

8. A cyanide-free aqueous electroplating bath comprising an aqueousammoniacal solution of an alkyl monoaminomonocarboxylic acid of from 2to 5 carbon atoms and cadmium sulfate in the molar ratio of from about1:1 to 2:1 in which the pH is up to about 10.

References Cited in the file of this patent UNITED STATES PATENTS2,377,228 Harford May 29, 1945

1. IN THE ELECTRODEPOSITION OF CADMIUM FROM AQUEOUS BATH, THEIMPROVEMENT WHICH COMPRISES ELECTRODEPOSITING CADMIUM FROM ACYANIDE-FREE AQUEOUS AMMONIACAL BATH OF PH UP TO ABOUT 11.2 IN WHICH ISDISSOLVED AN ALKYL MONOAMINOMONOCARBOXYLIC ACID OF FROM 2 TO 5 CARBONATOMS AND A CADMIUM SALT IN A MOLAR RATIO OF FROM ABOUT 1:1 TO 2:1.